Field of the Invention
The present invention relates to a method for producing a pressure-sensitive adhesive layer-carrying optical film having an anchor layer interposed between the film and a pressure-sensitive adhesive layer. Examples of the optical film include a polarizing film, a retardation plate, an optical compensation film, a brightness enhancement film, a reflection reducing film, any other surface-treating film, and any laminate in which two or more thereof are laminated onto each other.
Description of the Related Art
For example, in a liquid crystal display device, it is indispensable to arrange polarizing elements on both sides of its liquid crystal cell, respectively. Generally, in the liquid crystal display device, an organic EL (electro-luminescent) display device and the like, in light of an image forming manner thereof, a polarizing film is bonded thereto as such a polarizing element. In display panels such as liquid crystal panels and organic EL panels, various optical elements have been used besides a polarizing film to improve the display quality of their displays. A front plate has been used to protect image display devices such as a liquid crystal display device, an organic EL display device, a CRT and a PDP, give a high-class impression thereto, and discriminate a design thereof from other designs. In such image display devices, such as a liquid crystal display device and an organic EL display device, and members used together with the image display devices, such as a front plate, a surface-treating film is used, examples thereof including a retardation plate for preventing coloration, a viewing angle enlargement film for improving the viewing angle of the liquid crystal display, a brightness enhancement film for enhancing contrast on their display, a hard coat film used to give a scratch resistance onto their surface, an antiglare treatment film for preventing a surrounding image from being projected onto the image display devices, and a reflection reducing film such as an antireflective film or a low reflective film. These films are collectively named optical films.
When such an optical film is bonded onto a display panel such as a liquid crystal cell or organic EL panel, or onto a front plate thereof, a pressure-sensitive adhesive is usually used. About bonding between an optical film, and a display panel such as a liquid crystal cell or organic EL panel, a front plate, or an optical film, usually, a pressure-sensitive adhesive is used to cause the individual members to be bonded to adhere closely onto each other to decrease light loss. In such cases, it is general to use a pressure-sensitive adhesive layer-carrying optical film, in which a pressure-sensitive adhesive layer is beforehand laid onto a single side surface of an optical film, since the pressure-sensitive adhesive layer-carrying optical film has an advantage that no drying step is required for bonding and fixing the optical film, and other advantages.
An optical film is easily shrunken or expanded when heated or humidified. Thus, when adhesion between an optical film and an adhesive thereon is low, the optical film and the adhesive layer may be partially separated from each other, or may be peeled from each other. When a liquid crystal panel is used, particularly, for a car navigation system or any other article to be mounted onto an automobile, which requires the panel to have a higher endurance, a large shrinkage stress is applied onto the optical film (of the liquid crystal panel). Thus, the separation or peel is more easily caused. Specifically, although an optical film causes no problem in a reliability test made at about 80° C. for, for example, TVs, the film easily causes an inconvenience such as a separation or peel as described above in a reliability test made at about 95° C. for car navigation systems or any other article to be mounted onto an automobile. In a case where after a pressure-sensitive adhesive-layer-carrying optical film is bonded onto a liquid crystal display, the film is once peeled as needed and then is again bonded (or reworked) thereto, the following inconvenience is caused: when the adhesive force between the optical film and the pressure-sensitive adhesive is low, the pressure-sensitive adhesive unfavorably remains on the surface of the liquid crystal display device so that the rework is not efficiently attained. When pressure-sensitive adhesive layer-carrying optical film is handled in the step using the film such as a step of cutting the film, or carrying the film, an edge region of this film may contact a person or an article around the film. In this case, the pressure-sensitive adhesive may be removed in this region so that the liquid crystal panel easily undergoes an inconvenience such as display failure. In order to overcome such inconveniences, a method is carried out in which an anchor layer is applied onto an optical film and then a pressure-sensitive adhesive is applied thereonto to improve the adhesion between the optical film and the pressure-sensitive adhesive layer.
In the meantime, a pressure-sensitive adhesive layer as described above is required not to cause any inconvenience resulting from the pressure-sensitive adhesive when subjected to an endurance test by heating, humidifying and the like, which is usually made as an environment acceleration test. However, when an anchor layer is interposed between an optical film and a pressure-sensitive adhesive layer, a problem is caused that in an endurance test, a solvent crack is generated on the anchor-layer-formed side surface of the optical film. Even when no solvent crack is generated in a reliability test made at about 80° C. for, for example, TVs, a solvent crack may be remarkably generated, particularly, in a reliability test made at about 95° C. for car navigation systems or any other article to be mounted onto an automobile.
Patent Document 1 describes a pressure-sensitive adhesive layer-carrying optical film in which an anchor layer is interposed between the optical film and the pressure-sensitive adhesive layer, and the anchor layer is obtained by coating/drying a coating liquid for anchor layer containing a mixed solvent of water and an alcohol, and a polyamine compound. However, for this pressure-sensitive adhesive layer-carrying optical film, no investigation is concretely made about the composition of the coating liquid for anchor layer, and conditions for drying the liquid in order to solve the problem that a solvent crack is generated on the anchor-layer-formed side surface of the optical film in an endurance test.
Patent Document 2 describes a pressure-sensitive adhesive layer-carrying optical film in which an anchor layer is interposed between the optical film and the pressure-sensitive adhesive layer, and the anchor layer is obtained by coating/drying a coating liquid for anchor layer containing a mixed solvent of water and an oxazoline group-containing polymer, and specifically discloses an example in which about conditions for drying this coating liquid for anchor layer, the drying temperature and the drying period are set to 40 degrees and 120 seconds, respectively. Furthermore, Patent Document 3 describes a pressure-sensitive adhesive layer-carrying optical film in which an anchor layer is interposed between the optical film and the pressure-sensitive adhesive layer, and the anchor layer is obtained by coating/drying a coating liquid for anchor layer made of an aqueous solution containing a polyurethane resin and a water-soluble polythiophene based electroconductive polymer, and specifically discloses an example in which about conditions for drying this coating liquid for anchor layer, the drying temperature and the drying period are set to 80 degrees and 120 seconds, respectively. However, it has been made evident that under these drying conditions, there remains a room for making a further improvement from the viewpoint of preventing the generation of the above-mentioned solvent crack.
Patent Document 4 describes a pressure-sensitive adhesive layer-carrying optical film in which an anchor layer is interposed between the optical film and the pressure-sensitive adhesive layer, and the anchor layer is obtained by coating/drying a coating liquid for anchor layer containing ammonia and a water-dispersible polymer, and specifically discloses an example in which about conditions for drying this coating liquid for anchor layer, the drying temperature and the drying period are set to 50 degrees and 60 seconds, respectively. However, when the proportion of ammonia present in the anchor layer becomes large, the following is caused: in the case of using, for example, a polarizing film as the optical film in a high temperature and high humidity environment, the polarizing property of the polarizing film is varied, which affects the optical property so that the film may be unable to come to satisfy a high endurance in the high temperature and high humidity environment.
As described above, in the prior art, no attention has been paid to the problem that a solvent crack is generated on an anchor-layer-formed side surface of an optical film. Thus, in order to overcome this problem, a further investigation has been required.
[Patent Document 1] JP-A-2004-078143
[Patent Document 2] JP-A-2007-171892
[Patent Document 3] JP-A-2009-242786
[Patent Document 4] JP-A-2007-248485